Rotary tool

ABSTRACT

A hand held motor driven electrically powered tool, in particular a rotary hammer, comprising a spindle ( 40 ) for rotatingly driving a tool or bit ( 68 ), a spindle rotary drive train ( 14, 5, 10 ) for rotatingly driving the spindle ( 40 ) and an arrangement for detecting blocking events ( 16, 17 ). Blocking events occur when the tool or bit of the tool become rotationally fixed in the material being bored in which case the rotary drive on the spindle from the motor causes the tool housing to rotate in a user&#39;s grip. According to a first aspect of the invention an overload clutch ( 14 ) is provided in the spindle rotary drive train for transmitting rotary drive to the spindle below a predetermined torque and slipping above the predetermined torque arranged such that the overload clutch cuts off rotary drive to the spindle, for example by reducing the predetermined torque at which the overload clutch ( 14 ) slips when a blocking event is detected so as to cut off rotary drive to the spindle in response to a blocking event being detected. According to a second aspect of the invention there is provided a mode change mechanism ( 45, 47, 49, 43 ) for selectively disengaging a clutch ( 10, 7 ) so as to cut off the rotary drive to the spindle ( 40 ) and the clutch is disengaged when a blocking event is detected so as to cut off rotary drive to the spindle in response to a blocking event being detected.

The present invention relates to a hand held power tool with arotatingly driven tool or bit, in particular the present inventionrelates to electrically powered rotary hammering tools.

Rotary hammers will normally have a housing and a hollow cylindricalspindle mounted in the housing. The spindle allows insertion of theshank of a tool or bit, for example a drill bit or a chisel bit, intothe front end thereof so that it is retained in the front end of thespindle with a degree of axial movement The spindle may be a singlecylindrical part or may be made of two or more co-axial cylindricalparts, which together form the hammer spindle. For example, a front partof the spindle may be formed as a separate tool holder body forretaining the tool or bit.

Such hammers are provided with an impact mechanism which converts therotational drive from an electric motor to a reciprocating drive fordriving a piston, which may be a hollow piston, to reciprocate withinthe spindle. The piston reciprocatingly drives a ram by means of aclosed air cushion located between the piston and the ram. The impactsfrom the ram are transmitted to the tool or bit of the hammer,optionally via a beatpiece.

Rotary hammers can be employed in combination impact and drilling mode,and also in some cases in a drilling only mode, in which the spindle, ora forwardmost part of the spindle, and hence the bit inserted thereinwill be caused to rotate. In the combination impact and drilling modethe bit will be caused to rotate at the same time as the bit receivesrepeated impacts. A rotary drive mechanism transmits rotary drive fromthe electric motor to the spindle to cause the spindle, or a forwardmostpart thereof to rotate.

It is a problem with hand held power tools with a rotating bit thatduring use of the tool blocking events can sometimes occur, when the bitbecomes stuck in the workpiece in such a way that the bit can no longerrotate relative to the workpiece. In this case, the rotary drive to thebit causes the housing of the tool to rotate with respect to thestationary bit. It is known to detect blocking events, for example fromU.S. Pat. No. 5,914,882, U.S. Pat. No. 5,584,619, EP771,619 andGB2,086,277 and then once such an event is detected to arrest the rotarydrive to the bit. This can be done by braking the motor, which motorprovides rotary drive to the bit via a gear arrangement, but thisinvolves an inherent delay due to the time required to arrest the motor.Alternatively or additionally, the rotating spindle can be braked byengaging the spindle with a part of the hammer housing, in some way. Thepreferred way of cutting off rotary drive to the bit is by the use of areleasable clutch arrangement in the drive train between the motor ofthe hammer and the spindle.

The present invention aims to provide an improved design of clutch forcutting off rotary drive to the bit when a blocking event is detected.

According to an aspect of the present invention, there is provided anoverload clutch assembly for a power tool having a spindle forrotatingly driving a working member of the tool and a spindle rotarydrive train for rotatingly driving the spindle, the assembly comprising:

an overload clutch having a first mode in which rotary drive istransmitted to the spindle when a torque below a first predeterminedlevel is applied to the clutch, and transmission of rotary drive to thespindle is cut when a torque above said first predetermined level isapplied to the clutch, and at least one second mode in which rotarydrive is transmitted to the spindle when a torque below a respectivesecond predetermined level, lower than said first predetermined torque,is applied to the clutch, and transmission of rotary drive to thespindle is cut when a torque above said second predetermined level isapplied to the clutch; and

at least one actuator device for switching said overload clutch betweensaid first mode and at least one said second mode.

By providing an overload clutch assembly which has a first, highertorque, mode and at least one second, lower torque, mode, this providesthe advantage of enabling the tool to be selectively operated between ahigher torque mode and a safer, lower torque mode, in response todetection of blocking of the working member of the tool or on initialswitching on of the tool.

In a preferred embodiment, the overload clutch comprises at least onedriving gear adapted to be driven by a rotary drive train of the tool,at least one first driven gear for transmitting rotary drive to thespindle, a first coupling device for coupling at least one said drivinggear and at least one said first driven gear in said first mode when atorque below said first predetermined level is applied to the clutch andenabling decoupling of said driving gear and first driven gear when atorque above the first predetermined level is applied to the clutch, atleast one respective second driven gear for transmitting rotary drive tothe spindle, and at least one respective second coupling device forcoupling at least one said driving gear and at least one said seconddriven gear when a torque below the corresponding said secondpredetermined level is applied to the clutch in at least one said secondmode, and enabling decoupling of said driving gear and second drivengear when a torque above the second predetermined level is applied tothe clutch.

Preferably, at least one said coupling device couples at least onedriving gear and at least one corresponding driven gear by means of arespective set of locking elements acting between at least one saiddriving gear and at least one corresponding said driven gear.

A plurality of said locking elements may comprise ball bearings.

A plurality of said locking elements may comprise rollers.

The actuator device may be adapted to fix the rotational position of atleast one said second driven gear relative to at least one said firstdriven gear in said first mode.

In a preferred embodiment, at least one said first driven gear and atleast one said second driven gear are mounted to a common shaft, whereinat least one said second driven gear is non-rotatably mounted to saidshaft in the first mode thereof and is rotatable relative to said shaftin the second mode thereof.

The assembly may further comprise at least one detector device fordetecting blocking of a working member of the tool, wherein at least onesaid actuator device is adapted to switch said overload clutch to a saidsecond mode thereof in response to detection of blocking of said workingmember.

At least one said actuator device may be adapted to switch the overloadclutch to a said second mode thereof when the tool is switched on.

This provides the advantage of enabling the tool to automatically be setto a safer, low torque setting when the tool is initially switched on.

The assembly may further comprise at least one biasing device for urgingthe overload clutch to a said second mode thereof.

According to another aspect of the present invention, there is provideda switching assembly for switching an overload clutch assembly of apower tool between a first mode thereof and at least one second modethereof, the assembly comprising:

an actuator member movable between a first position corresponding to afirst mode, and at least one second position, corresponding to arespective second mode of the clutch assembly;

at least one connector member for actuating at least one actuator deviceof the clutch assembly in response to actuation of said actuator member;and

a latching device for releasably retaining said actuator member in atleast one said second position.

The assembly may further comprise at least one biasing device for urgingthe actuator device of the clutch assembly t o at least one said secondmode thereof.

At least one said biasing device may comprise a flexible lever.

At least one said connector member may comprise a cable.

Said latching device may comprise at least one releasable abutment forabutting a resilient member provided on said actuator member.

The assembly may further comprise a release device for releasing saidlatching device.

Said release device may comprise an electromagnet for displacing saidabutment.

Said release device may be adapted to be actuated on switching on of thetool.

According to a further aspect of the present invention, there isprovided a power tool comprising a spindle for rotatingly driving anoutput member of the tool;

a spindle rotary drive train for rotatingly driving the spindle; and

an overload clutch assembly as defined above.

The tool may further comprise a switching assembly as defined above.

According to a further aspect of the present invention, there isprovided a hand held, preferably motor driven, power tool, comprising:

a spindle for rotatingly driving a tool or bit;

a spindle rotary drive train for rotatingly driving the spindle;

an overload clutch in the spindle rotary drive train for transmittingrotary drive to the spindle below a predetermined torque and for cuttingtransmission of rotary drive above the predetermined torque; and

an arrangement for detecting blocking events; characterised in that theoverload clutch is arranged to cut off rotary drive to the spindle whena blocking event is detected.

Power tools with a rotatingly driven spindle, in particular if they arehighly powered tools, such as rotary hammers, generally have an overloadclutch in the drive train for rotatingly driving the spindle. Such anoverload clutch can help in the event of a blocking event, if the userhas a strong grip on the tool, because when the tool housing begins torotate, the torque required to rotatingly drive the spindle willincrease. If the torque increases to above the predetermined thresholdthen the overload clutch will stop transmitting rotary drive and drivewill no longer be transmitted to the spindle. The overload clutch alsoacts to reduce damage to components of the tool, for example the motorof the tool, when high torques are experienced. According to the firstaspect of the present invention an overload clutch is modified toprovide cutting off of rotary drive to the spindle in response to ablocking event being detected. The overload clutch may be located in anypart of the gear train from the motor of the tool to the spindle, andmay for example by the type of overload clutch known in the field whichis mounted around the spindle.

In order to cut off rotary drive to the spindle, the predeterminedtorque of the overload clutch above which transmission of rotary driveis cut off can be reduced in response to the detection of a blockingevent. The overload clutch may still provide the function of knownoverload clutches, depending on the level at which the predeterminedtorque is set when no blocking event is detected. In accordance with thepresent invention most types of overload clutch known for use in thedrive train of a rotary tool can be adapted to reduce the torque atwhich they stop transmitting rotary drive in response to the detectionof a blocking event. The torque at which the overload clutch stopstransmitting rotary drive may be reduced to substantially zero inresponse to the detection of a blocking event.

The present invention is particularly applicable to rotary hammers asthey are generally powerful tools and are known to experience problemsfrom blocking events. Such rotary hammers generally comprise a hammeringmechanism, generally located within the spindle, for generating repeatedimpacts on a tool or bit mounted at a forward end of the spindle. Thespindle is preferably made of as few parts as possible, but it mayinclude a separate tool holder portion located coaxially and forwardlyfrom the portion of the spindle in which the hammering mechanism ismainly located.

It is known to use arrangements for detecting blocking events which arepurely mechanical, for example using inertial masses, in which casethese arrangements can act mechanically on the overload clutch. Such amechanical arrangement may include an inertial mass pivotally mountedwithin the housing of the tool. According to one embodiment of thepresent invention, the arrangement for detecting blocking events maycomprise an inertial mass pivotally mounted within the tool housing andcomprising a latch for engaging an actuator of the overload clutch and aspring is provided for urging the actuator of the clutch into a cut offposition. These components are arranged such that when a blocking eventoccurs, the inertial mass pivots in the housing to disengage the latchfrom the actuator and the spring urges the actuator into the cut offposition in which the actuator causes the rotary drive to the spindle tobe cut off. This can provide a quick and accurate way of detecting ablocking event.

It is also known to use an electromechanical arrangement, which utilisesfor example, an inertial mass and senses the movement of the mass when ablocking event is occurring to generate an electrical output signal.

It is also known to detect blocking events electronically. For example,the arrangement for detecting blocking events may comprise a sensor, forexample an accelerometer, a torque sensor, a motor current or voltagesensor or other types of sensor known in the art for detecting blockingevents. The sensor senses an operational condition of the tool, forexample an accelerometer will detect vibrations of the tool and a torquesensor may detect a relative torque between components of the tool. Theoutputs from the sensor are fed into an electronic evaluation unit foranalysing the signals from the sensor and for generating an electricaloutput signal when a blocking event is detected. Examples of suchelectronic evaluation units are disclosed in U.S. Pat. No. 5,914,882,EP771,619 and U.S. Pat. No. 5,584,619.

Where the arrangement for detecting blocking events generates anelectrical output signal in response to the detection of a blockingevent, then the overload clutch may include an electromechanicalinterface, for example an electromagnet acting on a magnetic element,which interface is responsive to the output signal to reduce the torqueat which the overload clutch slips.

Generally an overload clutch will comprise a driven gear and a drivinggear and a coupling element, for example a resilient element or clutchballs biased by a resilient element, for coupling the driven gear anddriving gear below the predetermined torque and for enabling de-couplingof the driven gear and the driving gear above the predetermined torque.According to one embodiment of the present invention the arrangement fordetecting blocking events acts on the coupling element to cut off rotarydrive to the spindle when a blocking event is detected. The couplingelement may be a resilient element which couples the driven gear and thedriving gear via a set of locking elements mounted on one of the drivengear and the driving gear and engageable with the other of the drivengear and the driving gear in order to transmit rotary drivetherebetween. The arrangement for detecting blocking events may act tomove the coupling element, such as a resilient element, with respect tothe driven and driving gears in order to vary the torque at which theoverload clutch slips. Alternatively, the driven gear can be coupled tothe output of the overload clutch by a drive coupling and thearrangement for detecting blocking events acts on the drive coupling tocut off the transmission of rotary drive in response to the detection ofa blocking event.

Sometimes it is desirable, in particular in a rotary hammer, to havedifferent torques at which the rotary drive to the spindle is cut off,in different applications of the hammer. Therefore, in one embodiment ofthe present invention the overload clutch may have a first mode ofoperation in which the overload clutch transmits rotary drive to thespindle below a first predetermined torque and stops transmission ofrotary drive above the first predetermined torque, a second mode ofoperation in which the overload clutch transmits rotary drive to thespindle below a second predetermined torque, different from the firstpredetermined torque and stops transmission of rotary drive above thesecond predetermined torque, and a third mode of operation in which theoverload clutch cuts off rotary drive to the spindle when a blockingevent is detected.

The powered tool may be a rotary hammer, having a non-rotary mode and amode change mechanism of the hammer may be arranged to cause theoverload clutch to cut off rotary drive to the spindle, for example byreducing the torque at which the overload clutch stops transmittingrotary drive, when the non-rotary mode is selected. Thus, the overloadclutch according to the present invention, when incorporated in a rotaryhammer can additionally be used as part of the mode change mechanism ofthe hammer for stopping rotary drive to the hammer spindle when the modeis switched to the non-rotary mode.

According to a further aspect of the present invention there is provideda hand held, preferably motor driven electrically powered rotary hammer,comprising:

a spindle for rotatingly driving a tool or bit;

a hammering mechanism for generating repeated impacts on a tool or bitmounted at a forward end of the spindle;

a spindle rotary drive train for rotatingly driving the spindle;

a mode change mechanism for selectively disengaging a clutch in thespindle rotary drive train so as to cut off the rotary drive to thespindle; and

an arrangement for detecting blocking events; characterised in that theclutch is arranged to disengaged when a blocking event is detected.

Rotary hammers are well known with mode change mechanisms which are usedto selectively provide rotary drive to a spindle of the hammer. Forexample, in a drilling only mode or a rotary hammering mode of thehammer, the mode change mechanism acts to engage a clutch in the spindlerotary drive train so that rotary drive is transmitted to the spindle.In hammering only mode the mode change mechanism acts to disengage theclutch. Many such mode change mechanisms for selectively cutting offdrive to the spindle are known in the art and would be suitable for usein the present invention. In addition rotary hammers may also have apart of the mode change mechanism for selectively disengaging thehammering mechanism, as is well known in the art. According to thesecond aspect of the present invention the clutch in the spindle drivetrain acted on by the mode change mechanism to disengage rotary drive tothe spindle is also utilised to disengage rotary drive to the spindlewhen a blocking event is detected. Thus, the present invention has theadvantage of utilising the clutch for two purposes, mode change andcutting off rotary drive when blocking events occur.

The comments above in relation to the arrangement for detecting blockingevents also apply to this second aspect of the present invention.

In one embodiment the clutch includes a spindle drive gear arrangementaxially slideably mounted on the spindle and selectively engageable withpart of the spindle drive train in order to rotatingly drive thespindle. In this case the arrangement for detecting blocking events actson the spindle drive gear arrangement to move the spindle drive geararrangement axially along the spindle and out of engagement with thepart of the spindle drive train when a blocking event is detected.According to this embodiment, the mode change arrangement can also acton the spindle drive gear arrangement to move the spindle drive geararrangement axially along the spindle and out of engagement with thepart of the spindle drive train, when a non-rotary mode is selected.

The clutch may additionally be an overload clutch of the type describedabove in relation to the first aspect of the present invention.

According to a further aspect of the present invention there is provideda hand held, preferably motor driven electrically powered rotary hammer,comprising:

a spindle for rotatingly driving a tool or bit;

a hammering mechanism for generating repeated impacts on a tool or bitmounted at a forward end of the spindle;

a spindle rotary drive train for rotatingly driving the spindle;

an overload clutch in the spindle rotary drive train for transmittingrotary drive to the spindle below a predetermined torque and for cuttingtransmission of rotary drive above the predetermined torque;

a mode change mechanism for selectively cutting off the rotary drive tothe spindle; characterised in that the overload clutch has:

a first mode of operation in which the overload clutch transmits rotarydrive to the spindle below a first predetermined torque and stopstransmission of rotary drive above the first predetermined torque;

a second mode of operation in which the overload clutch transmits rotarydrive to the spindle below a second predetermined torque, different fromthe first predetermined torque, and stops transmission of rotary driveabove the second predetermined torque; and

a third mode of operation in which the overload clutch cuts off rotarydrive to the spindle in response to the mode change mechanism.

In one embodiment of this third aspect of the present invention overloadclutch may comprise a driven gear and a driving gear and a couplingelement for coupling the driven gear and driving gear below thepredetermined torques and for enabling de-coupling the driven gear andthe driving gear above the predetermined torques and a drive couplingfor coupling the driven gear to the output of the clutch, wherein themode change arrangement acts on the drive coupling to alter thepredetermined torque and to stop the transmission of rotary drive. Theremay be two driven gears and one of the driven gears can be coupled tothe output of the clutch via the drive coupling so that the overloadclutch has a first predetermined torque, the other or both of the drivengears can be coupled to the output of the clutch via the drive couplingso that the overload clutch has a second predetermined torque, differentfrom the first or neither of the driven gears can be coupled to theoutput of the clutch via the drive coupling so that the transmission ofrotary drive is stopped.

Preferred embodiments of the present invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings in which:

FIG. 1 shows a partially cutaway longitudinal cross section through arotary hammer of a first embodiment of the present invention;

FIG. 2 shows a longitudinal cross-section through a first embodiment ofan overload clutch of the rotary hammer of FIG. 1;

FIG. 3 shows a longitudinal cross-section through a second embodiment ofoverload clutch of the rotary hammer of FIG. 1;

FIG. 4 shows a partially cutaway longitudinal cross section through arotary hammer of a further embodiment of the present invention;

FIG. 5 shows a longitudinal cross-section through a rotary hammer of afurther embodiment of the present invention;

FIG. 6 shows a transverse cross-section of a mechanical blocking eventdetection arrangement of the rotary hammer of FIG. 5;

FIG. 7 shows a longitudinal cross-section through a further embodimentof an overload clutch of the rotary hammer of FIG. 5;

FIG. 8 shows a longitudinal cross-section through a further embodimentof the overload clutch of the rotary hammer of FIG. 5;

FIG. 9 shows a longitudinal cross-section through a further embodimentof an overload clutch suitable for use in the rotary hammer of FIGS. 1or 5;

FIG. 10 shows a broken apart perspective view of the components of theoverload clutch of FIG. 10;

FIG. 11 shows a fourth embodiment of an overload clutch suitable for usein the rotary hammer of FIGS. 1 or 5;

FIG. 12 is a cross sectional front elevation view of an overload clutchof a further embodiment of the invention;

FIG. 13 is a cross sectional side elevation view of a hammer of afurther embodiment of the present invention;

FIG. 14 is a partially cut away perspective view of an overload clutchassembly of a further embodiment of the present invention;

FIG. 15 is an exploded perspective view of the clutch assembly of FIG.14;

FIGS. 16A to C show details of a first driven gear of the clutchassembly of FIGS. 14 and 15;

FIGS. 17A to 17C show details of a second driven gear of the clutchassembly of FIGS. 14 and 15;

FIG. 18 shows a schematic perspective view of a switching assembly ofthe hammer of FIG. 13;

FIG. 19 shows a schematic partially cutaway view of part of the hammerof FIG. 14;

FIG. 20 is a schematic perspective view of a latching arrangement of theswitching assembly of FIG. 18;

FIG. 21 is a detailed view of a release mechanism for the latchingmechanism of FIG. 20; and

FIG. 22 is a logic diagram for the release mechanism of FIG. 21.

In the Figures like parts are identified by like numerals.

The hammer shown in FIG. 1 comprises an electric motor (2), an spindledrive train and a crank drive arrangement which are housed within ametal gear housing (not shown) surrounded by a plastic housing (4). Arear handle housing incorporating a rear handle (6) and a trigger switcharrangement (8) is fitted to the rear of the housing (4). A cable (notshown) extends through a cable guide and connects the motor to anexternal electricity supply. Thus, when the cable is connected to theelectricity supply and the trigger switch arrangement (8) is depressedthe motor (2) is actuated to rotationally drive the armature of themotor. The metal gear housing is made from magnesium with steel insertsand rigidly supports the components housed within it.

A driving gear (9) is press fitted onto the motor pinion (3) and hasteeth which engage the teeth of a driving gear (12) of an overloadclutch arrangement (14) to rotatingly drive the driving gear (12). Thedriving gear (12) rotatingly drives a driven gear (13) of the overloadclutch arrangement (14) when the torque transmitted between the twogears (12, 13) is below a predetermined threshold and if no blockingevent is detected. The driven gear (13) is press fit onto a spindledrive shaft (5), formed with a bevel pinion (7) at its end remote fromthe driven gear wheel (13). The bevel pinion meshes with a beveledspindle drive gear (10) which drive gear is non-rotatably mounted on thespindle (40). The overload clutch arrangement (14) is described in moredetail below with respect to FIGS. 2 and 3.

The teeth of the driving gear (9) also engage the teeth of a crank drivegear (20) to rotatingly drive the drive gear (20). The drive gear (20)is non-rotatably mounted on a crank drive spindle (22) which spindle isrotatably mounted within the gear housing. A crank plate (30) isnon-rotatably mounted at the end of the drive spindle remote from thedrive gear (20), which crank-plate is formed with an eccentric bore forhousing an eccentric crank pin (32). The crank pin (32) extends from thecrank plate into a bore at the rearward end of a con-rod or crank arm(34) so that the con-rod (34) can pivot about the crank pin (32). Theopposite forward end of the con-rod (34) is formed with a bore throughwhich extends a trunnion pin (36) so that the con-rod (34) can pivotabout the trunnion pin. The trunnion pin (36) is fitted to the rear of apiston (38) by fitting the ends of the trunnion pin (36) into receivingbores formed in a pair of opposing arms, which arms extend to the rearof the piston (38). The piston is reciprocally mounted in a cylindricalhollow spindle (40) so that it can reciprocate within the hollowspindle. An O-ring seal is fitted in an annular recess formed in theperiphery of the piston (38) so as to form an air tight seal between thepiston (38) and the internal surface of the hollow spindle (40).

Thus, when the motor (2) is actuated, the armature pinion (3) rotatinglydrives the driving gear (9) and the driving gear rotatingly drives thecrank drive spindle (22) via the drive gear (20). The drive spindlerotatingly drives the crank plate (30) and the crank arm arrangementcomprising the crank pin (32), the con-rod (34) and the trunnion pin(36) convert the rotational drive from the crank plate (30) to areciprocating drive to the piston (38). In this way the piston (38) isreciprocatingly driven back and forth along the hollow spindle (40),when the motor (2) is actuated by depression of the trigger switch (8).The driving gear (9) also drives the driving gear (12) of the clutcharrangement (14) which drives the driven gear (13) of the clutcharrangement. The driven gear (13) of the clutch arrangement rotatinglydrives the spindle drive shaft (5) which rotatingly drives the spindledrive gear (10) and thus the spindle (40) via the bevel pinion (7).

A ram (58) is located within the hollow spindle (40) forwardly of thepiston (38) so that it can also reciprocate within the hollow spindle(40). An O-ring seal is located in a recess formed around the peripheryof the ram (58) so as to form an air tight seal between the ram (58) andthe spindle (40). In the operating position of the ram (58), with theram located rearward of venting bores (not shown) in the spindle aclosed air cushion (44) is formed between the forward face of the piston(38) and the rearward face of the ram (58). Thus, reciprocation of thepiston (38) reciprocatingly drives the ram (58) via the closed aircushion (44). When the hammer enters idle mode (ie. when the hammer bitis removed from a workpiece), the ram (58) moves forwardly, past theventing bores. This vents the air cushion and so the ram (58) is nolonger reciprocatingly driven by the piston (38) in idle mode, as iswell known in the art.

A beatpiece (64) is guided so that it can reciprocate within forwardportion of the spindle. A bit or tool (68) can be releasably mountedwithin a tool holder (66) so that the bit or tool (68) can reciprocateto a limited extent within a tool holder portion of the spindle. Whenthe ram (58) is in its operating mode and is reciprocatingly driven bythe piston (38) the ram repeatedly impacts the rearward end of thebeatpiece (64) and the beatpiece (64) transmits these impacts to therearward end of the bit or tool (68) as is known in the art. Theseimpacts are then transmitted by the bit or tool (68) to the materialbeing worked.

In the arrangement in FIG. 1, an operational condition of the rotaryhammer is monitored by a sensor, such as an angular accelerometer (16).The signals from the sensor (16) are transmitted via an input interfaceto an electronic evaluation unit, which may be formed as amicrocontroller (17). The micro-controller analyses the signals from theaccelerometer (16) and is programmed to generate an output signal when ablocking event is about to occur. For example, the arrangement of thetype described in U.S. Pat. No. 5,584,619, U.S. Pat. No. 5,914,882 or EP771,619 can be used to generate an output signal when a blocking eventis about to occur. The warning signal triggers a circuit (18) powered bythe power supply to the motor (2) of the hammer. The circuit (18), whentriggered supplies electric current to an electromagnet (19) whichcauses the clutch arrangement (14) to disengage in order to interruptthe drive from the driving gear (9) to the spindle drive shaft (5).

One embodiment of an overload clutch arrangement suitable for use in thearrangement of FIG. 1 is shown in FIG. 2. The driving gear (12) of theoverload clutch arrangement (14) is rotationally mounted within thehammer housing via a bearing (15). The driving gear (12) is mounted torotate about an actuating shaft (21), which actuating shaft is axiallyslideable with respect to the driving gear. The driven gear (13) of theoverload clutch arrangement (14) is rotationally mounted within thehammer housing via a bearing (23). The driven gear (13) is also mountedto rotate about the axially slideable actuating shaft (21) and is formedwith a bore (24) for axially slidably receiving a first end of theactuating shaft (21).

Rotary drive is transmitted between the driving gear (12) and the drivengear (13) of the overload clutch arrangement (14) via a plurality oflocking balls (25). The driving gear (12) is formed with a cylindricalsleeve portion (12 a) which extends within a cylindrical sleeve portion(13 a) of the driven gear (13). The locking balls (25) are mounted incorresponding holes radially formed through the cylindrical sleeveportion (12 a). The balls are mounted so as to be shiftable in a radialdirection. The actuating shaft (21) has an increased diameter portion(21 a) which is slideable within the cylindrical sleeve portion (12 a)of the driving gear (12). A cylindrical sleeve (26) is mounted on theincreased diameter portion (21 a) of the actuating shaft, co-axial withand in the space between the actuating shaft (21) and the cylindricalsleeve portion (12 a). The cylindrical sleeve (26) is resilient and actsto bias the locking balls (25) into a radially outward position in whichthe locking balls engage a corresponding set of pockets (13 b) formed inthe radially inwardly facing surface of the cylindrical sleeve portion(13 a) of the driven gear. The pockets (13 b) (see left hand side ofFIG. 2) are separated by a set of sloped ridges. When the locking balls(25) engage the pockets (13 b) rotary drive is transmitted between thedriving gear (12) and the driven gear (13) and rotational drive istransmitted via the spindle drive shaft (5) to the spindle (40).

When the actuating shaft (21) is the position shown in FIG. 2, theclutch arrangement (14) acts as an overload clutch. Below apredetermined torque, the resilient sleeve (26) biases the locking balls(25) into engagement with the pockets (13 b) in the driven gear (13) tothereby transmit rotation from the driving gear (12) to the driven gear(13). Thus, rotary drive is transmitted to the spindle (40) via thespindle drive shaft (5). However, above the predetermined torque, thebiasing force from the resilient sleeve (26) becomes insufficient tobias the locking balls (25) into the pockets (13 b) in the driven gear(13) and the balls can move radially inwardly to ride up the slopes andover the ridges between the pockets (13 b) (see right hand side of FIG.2). Thus, the driven gear (13) rotates with respect to the driving gear(12) and rotary drive to the spindle drive shaft (5) and thus to thespindle (40) is cut-off.

The overload clutch arrangement of FIG. 2, also acts to cut-off rotarydrive to the spindle (40) when a blocking event is detected. When thesignals from the accelerometer (16) are analysed by the microprocessor(17) so that the microprocessor determines that a blocking event isoccurring, an output signal is output from the microprocessor into thecircuit (18). This causes the circuit (18) to apply current to anelectromagnet (19). The electromagnet is mounted in the hammer housing(4) so that it surrounds an end of the actuating shaft (21) remote fromthe end received in the bore (24) of the driven gear (13). A magneticelement (27) is mounted on the end of the actuating shaft (21)surrounded by the electromagnet (19). When current is supplied to theelectromagnet (19) a magnetic force is created between the electromagnet(19) and the magnetic element (27) which draws the magnetic elementdownwardly in the direction of the arrow A of FIG. 2. The actuatingshaft (21) moves downwardly until the increased diameter portion (21 a)of the actuating shaft (21) abuts the base of the cylindrical sleeve (12a) of the driving gear (12). This movement of the actuating shaft (21)moves the resilient sleeve (26) downwardly in the direction of the arrowA until only the upper edge of the resilient sleeve engages the lockingballs (25). Thus, the radially outwardly biasing force on the lockingballs (25) from the biasing sleeve (26) is significantly reduced and thelocking balls move radially inwardly, out of the pockets (13 a) in thecylindrical sleeve (13 a) of the driven gear (13). Thus, rotary drive tothe driven gear (13) and so to the spindle (40) via the spindle driveshaft (5) is cut off. Accordingly, as soon as current is supplied to theelectromagnet (19) from the circuit (18), the clutch arrangement (14) isdisengaged and no further rotary drive is transmitted via the clutcharrangement (14) to the spindle (40). In this way the potentiallydangerous consequences of a blocking event are avoided. A return springcan be provided to return the actuating shaft (21) to its originalposition.

It should be noted that disengagement of the clutch arrangement (14) ofFIG. 2 could also be used to switch the hammer into its hammering onlymode position in which no rotary drive is transmitted to the spindle(40). This mode change could be performed electromechanically using theelectromagnet (19) to move the actuating shaft (21) or could be donemechanically by utilising mechanical means to shift the actuating shaft.

A second embodiment of an overload clutch arrangement suitable for usein the arrangement of FIG. 1 is shown in FIG. 3. The driving gear (12),driven gear (13) and actuating shaft (21) are mounted in the housing asdescribed for the FIG. 2 embodiment. Rotary drive is transmitted betweenthe driving gear (12) and the driven gear (13) of the overload clutcharrangement (14) via a plurality of locking balls (25). The driving gear(12) is formed with a cylindrical sleeve portion (12 a) which extendswithin a cylindrical sleeve portion (13 a) of the driven gear (13). Thelocking balls (25) are mounted in corresponding holes formed through thecylindrical sleeve portion (12 a) so as to be shiftable in a radialdirection. The actuating shaft (21) has an increased diameter portion(21 a) which is slideable within the cylindrical sleeve portion (12 a)of the driving gear (12). A cylindrical sleeve (26) is located withinthe cylindrical sleeve portion (12 a), co-axial with and in the spacebetween the actuating shaft (21) and the cylindrical sleeve portion (12a). The cylindrical sleeve (26) is resilient and the increased diameterportion (21 a) of the actuating shaft bears on the internal surface ofthe resilient sleeve (24) to reinforce a biasing force from theresilient sleeve which biases the locking balls (25) into a radiallyoutward position in which the locking balls engage a corresponding setof pockets (13 b) formed in the radially inwardly facing surface of thecylindrical sleeve portion (13 a) of the driven gear. The pockets (13 b)are separated by a set of sloped ridges. When the locking balls (25)engage the pockets (13 b) (see left hand side of FIG. 3) rotary drive istransmitted between the driving gear (12) and the driven gear (13) androtational drive is transmitted via the spindle drive shaft (5) to thespindle (40).

When the increased diameter portion (21 a) of the actuating shaft (21)is the position shown in dotted lines (a) FIG. 3, the clutch arrangement(14) acts as an overload clutch. Below a predetermined torque, theresilient sleeve (26), reinforced by the increased diameter portion (21a) of the actuating shaft (21) biases the locking balls (25) intoengagement with the pockets (13 b) in the driven gear (13) to therebytransmit rotation from the driving gear (12) to the driven gear (13).Thus, rotary drive is transmitted to the spindle (40) via the spindledrive shaft (5). However, above the predetermined torque, the biasingforce from the resilient sleeve (26) becomes insufficient to bias thelocking balls (25) into the pockets (13 b) in the driven gear (13) andthe balls can move radially inwardly (see right hand side of FIG. 3) toride up the slopes and over the ridges between the pockets (13 b). Thus,the driven gear (13) rotates with respect to the driving gear (12) androtary drive to the spindle drive shaft (5) and thus to the spindle (40)is cut-off.

The overload clutch arrangement of FIG. 3, also acts to cut-off rotarydrive to the spindle (40) when a blocking event is detected. Anelectromagnet (19) is mounted in the hammer housing (4) so that itsurrounds an end of the actuating shaft (21) remote from the endreceived in the bore (24) of the driven gear (13). A magnetic element(27) is mounted on the end of the actuating shaft (21) surrounded by theelectromagnet (19). When current is supplied to the electromagnet (19) amagnetic force is created between the electromagnet (19) and themagnetic element (27) which draws the magnetic element downwardly in thedirection of the arrow A of FIG. 3 into the position shown in dottedlines (b) in FIG. 3. The actuating shaft (21) moves downwardly until theincreased diameter portion (21 a) of the actuating shaft (21) abuts arim of the base of the cylindrical sleeve (12 a) of the driving gear(12). This movement of the actuating shaft (21) moves the increaseddiameter portion (21 a) of the actuating shaft (21) downwardly in thedirection of the arrow A until it bears against only the lower edge ofthe resilient sleeve (26). Thus, the radially outwardly biasing force onthe locking balls (25) from the biasing sleeve (26) is significantlyreduced and the locking balls move radially inwardly (see right handside of FIG. 3), out of the pockets (13 a) in the cylindrical sleeve (13a) of the driven gear (13). Thus, rotary drive to the driven gear (13)and so to the spindle (40) via the spindle drive shaft (5) is cut off.Accordingly, as soon as current is supplied to the electromagnet (19)from the circuit (18), the clutch arrangement (14) is disengaged and nofurther rotary drive is transmitted via the clutch arrangement (14) tothe spindle (40). In this way the potentially dangerous consequences ofa blocking event are avoided. A return spring can be provided to returnthe actuating shaft (21) to its original position.

It should be noted that disengagement of the clutch arrangement (14) ofFIG. 3 could also be used to switch the hammer into its hammering onlymode position in which no rotary drive is transmitted to the spindle(40). This mode change could be performed electromechanically using theelectromagnet (19) to move the actuating shaft (21) or could be donemechanically by utilising mechanical means to shift the actuating shaft.

The cut off of rotary drive to the spindle (40) is achieved by utilisingan already existing component in the drive train to the hammermechanism, ie. the overload clutch. In the embodiments of FIGS. 1 to 3,the overload clutch arrangement is altered to enable it also to cut offrotary drive to the spindle (40) by reducing the torque at which theoverload clutch slips when a blocking event is detected.

A rotary hammer according to a second aspect of the present invention isshown in FIG. 4. The hammer in FIG. 4 differs from that in FIG. 1 inthat the rotary drive train from the motor (2) to the spindle (40) isdifferent. The drive from the driving gear (9) is transmitted to thespindle drive shaft (5) via a gear wheel (41) press fit onto the spindledrive shaft. At its end remote from the gear wheel (41) the spindledrive shaft (5) is formed with a pinion (7) which is engageable with aspindle drive gear (10). The spindle drive gear (10) is mounted on asliding sleeve (43), which sliding sleeve is axially slideably butrotationally fixedly mounted on the spindle (40). The mounting of thespindle drive gear (10) on the slider sleeve (43) may be a rotationallyand axially fixed mounting, as shown in FIG. 4, arranged such thatrotation of the spindle drive gear (10) rotatingly drives the slidingsleeve (43) and so rotatingly drives the spindle (40). Alternatively,this mounting may be via an overload clutch as is known in the art,arranged such that rotation of the spindle drive gear (10) rotatinglydrives the sliding sleeve (43) to rotatingly drive the spindle (40)below a predetermined torque and slips relative to the sliding sleeveabove a predetermined torque, so that above the predetermined torque thespindle (40) is no longer rotatingly driven.

The hammer shown in FIG. 4, has two modes hammering only mode and rotaryhammer mode. FIG. 4 shows the rotary hammer mode in which the pinion (7)of the spindle drive shaft (5) engages the spindle drive gear (10),which is a bevel gear, to rotatingly drive the spindle (40) via thesliding sleeve (43). Thus, a tool (68) mounted within the forward end ofthe spindle is rotatingly driven via the spindle and simultaneouslyreceives repeated impacts from the beatpiece (64) of the hammeringmechanism. A mode change knob (45) is rotationally mounted within thehammer housing (4) and is formed with an eccentric pin (47), which pinextends into the hammer housing (4). The eccentric pin (47) is receivedwithin a recess at the first rearward end of a mode change linkage (49).The forward end of the mode change linkage is formed with a finger (49a) which finger is engageable with a raised peripheral rim at theforward end of the sliding sleeve (43). In the position shown in FIG. 4,the mode change knob is turned to its rotary hammer mode position andthe finger (49 a) of the linkage (49) does not engage the sliding sleeve(43). The sliding sleeve is thus biased by a helical spring (50) intoits rearward rotary hammer mode position, as shown in FIG. 4. Thehelical spring (50) is mounted around the spindle (40) and acts betweena circlip (51) on the spindle at the forward end of the spring and thesliding sleeve (43) at the rearward end of the spring, in order to biasthe sliding sleeve rearwardly.

The hammer can be changed into a hammering only mode by rotating themode change knob (45) so that the eccentric pin (47) moves to the leftin FIG. 4. The eccentric pin (47) engages the mode change linkage (49)to move it forwardly (to the left in FIG. 4). The finger (49 a) of themode change linkage engages the rim of the sliding sleeve (43) to urgeit forwardly against the biasing force of the spring (50). The spindledrive gear (10) is axially fixed on the sliding sleeve and so thespindle drive gear (10) moves forwardly with the sliding sleeve (43) outof engagement with the pinion (7) of the spindle drive shaft, and sorotary drive to the spindle is shut off. In the forward position of thespindle drive gear (10) the spindle drive gear can engage a set ofcooperating teeth mounted within the housing (4) to lock the spindleagainst rotation in its hammering only mode, as is well known in theart.

On turning the mode change knob back into rotary hammering modeposition, as shown in FIG. 4, the sliding sleeve (43) is urgedrearwardly back into the FIG. 4 position by the spring (50).

The rotary hammer shown in FIG. 4 has the same blocking event detectingarrangement comprising an accelerometer (16), a microprocessor (17),circuit (18) and electromagnet (19), as described above in relation toFIG. 1, except that the electromagnet (19) surrounds the spindle drivegear (10), and the circuit (18) is repositioned between themicroprocessor and the electromagnet. The spindle drive gear (10) and/orthe sliding sleeve (43) are formed at least partly of a magneticmaterial. Thus, when a blocking event is detected the circuit (18)supplies current to the electromagnet (19) and this causes a magneticforce between the electromagnet (19) and the magnetic material in thespindle drive gear and/or sliding sleeve in order to move the slidingsleeve (43) and spindle drive gear (10) forwardly (to the left in FIG.4) against the biasing force of the spring (50) and out of engagementwith the pinion (7) of the spindle drive shaft (5). In this way, when ablocking event is detected rotary drive is cut-off between the motor (2)and the spindle (40) by disengaging the driving connection between thespindle drive shaft (5) and the spindle drive gear (10). Thisarrangement requires only the addition of the electromagnet (19) and ofmagnetic material to the spindle drive gear (10) and/or sliding sleeve(43) in order to cut off rotary drive to the spindle. No furthercomponents or changes are required to be made to components alreadyexisting rotary hammer components. In the FIG. 4 embodiment, this isachieved by using the already existing mode change components forswitching the rotary drive to the hammer spindle (40) on and off.

Alternatively, the rotary hammer of FIG. 4 can be designed to have anadditional drilling only mode in which the hammer drive mechanism isshut off, as is well known in the art.

The rotary hammer shown in FIG. 5 is similar to that of FIG. 1, exceptthat it has a purely mechanical arrangement for detecting blockingevents, as opposed to an electromechanical arrangement for detectingblocking events. Instead of the accelerometer (16), micro-controller(17), circuit (18) and electromagnet (19) of the FIG. 1 embodiment, thehammer of FIG. 5 has the mechanical arrangement shown, from the front,in FIG. 6.

The arrangement for detecting blocking events shown in FIGS. 5 and 6comprises an inertial mass (72) which is formed at the lower end of alever (74), the upper end of which lever (74) is pivotally mounted withrespect to the hammer housing, via pivot pin (76) so that the mass (72)and lever (74) are pivotal about an axis (80) extending parallel to thespindle axis (78). The mass is connected via a spring (82) to a mountingblock (84) which. mounting block is rigidly mounted with respect to thehammer housing (4). A first end of the spring (82) is fixed to themounting block (84) and a second end of the spring (82) is fixed to themass (72). As the hammer is operated, the mass vibrates, and so pivotsabout the pivot pin (76) due to the vibrations occurring from theoperation of the hammer. The spring (82) is arranged to damp thevibration of the mass (72) and so minimise the extent of the pivoting ofthe mass (72) about the pivot pin (76) during normal operation of thehammer. The upper end of the lever (74), above the pivot pin (76) isformed with a latching ledge (86), which during normal operation of thehammer engages with a facing latching ledge (88) formed at the lower endof an actuating shaft (21) of a clutch arrangement (14), discussed belowin relation to FIGS. 7 and 8. The actuating shaft (21) of the clutcharrangement (14) is slideably mounted for movement in the direction ofarrow (X) within components of the clutch and within a bushing (90). Theactuating shaft (21) is biased by a strong spring (92), upwards, in thedirection of the clutch arrangement (14).

During normal operating of the hammer, the pivoting movement of the mass(72) about the pivot pin (76) is limited by the damping action of thespring (82). However, when a blocking event occurs, the bit (68) becomesrotationally fixed in the material being worked and the hammer housingis rotatingiy driven about the bit (68) by the motor (2) via the spindlerotary drive arrangement. This causes the lower part of the hammerhousing (4 a) to rotate, with a very high acceleration, about thespindle axis (78) so that said lower part moves in a direction out ofthe paper of FIG. 5. The inertia of the mass (72) causes the mass topivot about the pin (76) in a direction into the paper in FIG. 5, ie. inthe direction of the arrow (Y) in FIG. 6, so as to compress the spring(82). The upper end of the lever (74) above the pin (76) pivots in thedirection of the arrow (Z) with respect to the pivot pin (76), whichcauses the latching ledge (86) of the lever (74) to disengage thelatching ledge (88) of the actuating shaft (21). The strong spring (92)is then able to urge the actuating shaft (21) to move upwardly to causethe clutch (14) to disengage, as is described below in relation to FIGS.7 and 8, and so rotary drive from the motor (2) to the spindle (40) iscut off and the housing (4) is not rotatingly driven any further.

A lever (94) is provided on the actuating shaft (21) to re-set theblocking event detection arrangement of FIG. 6 after the rotary drivehas been cut off in response to the detection of a blocking event. Thelever (94) can extend outside of the hammer housing (4) or can engage asliding knob actuable from the outside of the housing (4), so that thelever (94) and thus the actuating shaft can be pulled downwardly. As theshaft (21) is pulled downwardly, against the force of the strong spring(92), chamfered outer edges (96, 98) formed on the actuating shaft (21)and the upper end of the lever (74) engage to pivot the lever (74) aboutthe pivot pin (76) in the direction of the arrow (Z) against the biasingforce of the spring (82) so as to re-engage the latching ledges (86, 88)of the actuating shaft (21) and the lever (74).

An overload clutch arrangement suitable for use in the hammer of FIG. 5is shown in FIG. 7. The driving gear (12), driven gear (13) andactuating shaft (21) are mounted in the housing as described for theFIG. 2 embodiment, with an extra guiding bushing (90) for slideablyguiding the actuating shaft (21) as described above in relation to FIGS.5 and 6. Rotary drive is transmitted between the driving gear (12) andthe driven gear (13) of the overload clutch arrangement (14) via aplurality of locking balls (25). The driving gear (12) is formed with acylindrical sleeve portion (12 a) which extends within a cylindricalsleeve portion (13 a) of the driven gear (13). The locking balls (25)are mounted in corresponding holes formed through the cylindrical sleeveportion (12 a) so as to be shiftable in a radial direction. Theactuating shaft (21) has an reduced diameter portion (21 b) which isslideable within the cylindrical sleeve portion (12 a) of the drivinggear (12). A plurality of spring elements (100) are circumferentiallyspaced around the actuating shaft (21) and are pivotably mounted withrespect to the reduced diameter portion (21 b) via balls (102). Eachspring element comprises a helical spring (106) mounted within a guidejacket (104) and extends radially with respect to the actuating shaft(21) between the balls (102) and the locking balls (25). Each ball (102)is received within an associated pocket in the reduced diameter portion(21 b) and a pocket formed at the radially inner end of the resilientjacket (104) of an associated spring element (100). This enables thelocking elements (100) to pivot between the positions shown on the lefthand side and the right hand side of FIG. 7. In the position shown inthe left hand side of FIG. 7, the spring elements (100) bias the lockingballs (25) into a radially outward position in which the locking ballsengage a corresponding set of pockets (13 b) formed in the radiallyinwardly facing surface of the cylindrical sleeve portion (13 a) of thedriven gear. The pockets (13 b) are separated by a set of sloped ridges.When the locking balls (25) engage the pockets (13 b) rotary drive istransmitted between the driving gear (12) and the driven gear (13) androtational drive is transmitted via the spindle drive shaft (5) to thespindle (40).

When the reduced diameter portion (21 b) of the actuating shaft (21) isthe position shown in the left hand side of FIG. 7, the clutcharrangement (14) acts as an overload clutch. Below a predeterminedtorque, the spring elements (100) bias the locking balls (25) intoengagement with the pockets (13 b) in the driven gear (13) to therebytransmit rotation from the driving gear (12) to the driven gear (13).Thus, rotary drive is transmitted to the spindle (40) via the spindledrive shaft (5). However, above the predetermined torque, the biasingforce from the spring elements (100) become insufficient to bias thelocking balls (25) into the pockets (13 b) in the driven gear (13) andthe balls can move radially inwardly to ride up the slopes and over theridges between the pockets (13 b). Thus, the driven gear (13) rotateswith respect to the driving gear (12) and rotary drive to the spindledrive shaft (5) and thus to the spindle (40) is cut-off.

The overload clutch arrangement of FIG. 7, also acts to cut-off rotarydrive to the spindle (40) when a blocking event is detected. Asdescribed above in relation to FIGS. 5 and 6, when a blocking eventoccurs, the inertial mass (72) pivots in the direction (Y) causing theupper end of the lever (74) to pivot in direction (Z) thus causing thelatching ledges (86, 88) on the lever (74) and actuating shaft (21) todisengage. The strong spring (92), which is axially fixed at its lowerend to an increased diameter portion (21 c) and is axially fixed at itsupper end to the driving gear (12), acts to pull the increased diameterportion (21 c) of the actuating shaft, upwardly and so pulls theactuating shaft upwardly into the position shown on the right hand sideof FIG. 7. This movement of the actuating shaft (21) moves the decreaseddiameter portion (21 b) of the actuating shaft (21) upwardly and causesthe spring elements to pivot about the pivot balls (102). This pivotingof the spring elements (100) leads to an extension of the springs (104)which reduces the biasing forces from the spring elements (100) on thelocking balls (25). In this way, the radially outwardly biasing force onthe locking balls (25) from the spring elements (100) is significantlyreduced and the locking balls move radially inwardly, out of the pockets(13 a) in the cylindrical sleeve (13 a) of the driven gear (13). Thus,rotary drive to the driven gear (13) and so to the spindle (40) via thespindle drive shaft (5) is cut off. Accordingly, as soon as the latchingledges (86, 88) of the lever (74) and clutch actuating shaft (21) aredisengaged no further rotary drive is transmitted via the clutcharrangement (14) to the spindle (40). In this way the potentiallydangerous consequences of a blocking event are avoided.

A second embodiment of an overload clutch arrangement suitable for usein the hammer of FIG. 5 is shown in FIG. 8. The driving gear (12),driven gear (13) and actuating shaft (21) are mounted in the housing asdescribed for the FIG. 2 embodiment, with an extra guiding bushing (90)for slideably guiding the actuating shaft (21) as described above inrelation to FIGS. 5 and 6. Rotary drive is transmitted between thedriving gear (12) and the driven gear (13) of the overload clutcharrangement (14) via a plurality of locking balls (25). The driving gear(12) is formed with a cylindrical sleeve portion (12 a) which extendswithin a cylindrical sleeve portion (13 a) of the driven gear (13). Thelocking balls (25) are mounted in corresponding holes formed through thecylindrical sleeve portion (12 a) so as to be shiftable in a radialdirection. The actuating shaft (21) is formed with three increaseddiameter annulae (121 a to 121 c), the middle of which (121 b) is ofreduced diameter, as compared to the others. The annulae (121 a to c)are slideable within the cylindrical sleeve portion (12 a) of thedriving gear (12). A first plurality of springs (110) arecircumferentially spaced around the actuating shaft (21) and extendradially with respect to the actuating shaft (21) between the lower twoannulae (121 c and 121 b) from the actuating shaft to an associatedguide element (112). A second plurality of springs (114) arecircumferentially spaced around the actuating shaft (21) and extendradially with respect to the actuating shaft (21) between the upper twoannulae (121 a and 121 b) from the actuating shaft to the associatedguide element (112). The radially outer end of each spring is mountedaround an associated radially inwardly extending peg (116) formed on theassociated guide element (112). Each guide element is formed with twopegs (116), an upper peg for engaging the end of one of the springs(114) and a lower peg for engaging the end of one of the springs (110)directly below said one of the springs (114). The first plurality ofsprings (110) exert a weaker radially outward biasing force than thesecond plurality of springs (114). Depending on the axial position ofthe actuating shaft (21), either the strong springs (114) or the weaksprings (110) bias the locking balls (25) radially outwardly via theguide elements (112).

When the latching ledges (86, 88) are engaged and the spring (92) isextended the annulus (121 c) is moved downwardly from its position inFIG. 8 and abuts the base of the driving gear sleeve (12). In thisposition the strong springs (114) are radially inwardly of the lockingballs (25) and the clutch arrangement (14) acts as an overload clutch.Below a predetermined torque, the strong springs (114) bias the lockingballs (25) into engagement with the pockets (13 b) in the driven gear(13) to thereby transmit rotation from the driving gear (12) to thedriven gear (13). Thus, rotary drive is transmitted to the spindle (40)via the spindle drive shaft (5). However, above the predeterminedtorque, the biasing force from the springs (114) become insufficient tobias the locking balls (25) into the pockets (13 b) in the driven gear(13) and each guide element (112) pivots inwardly and the balls can moveradially inwardly to ride up the slopes and over the ridges between thepockets (13 b). Thus, the driven gear (13) rotates with respect to thedriving gear (12) and rotary drive to the spindle drive shaft (5) andthus to the spindle (40) is cut-off.

The overload clutch arrangement of FIG. 8, also acts to cut-off rotarydrive to the spindle (40) when a blocking event is detected. Asdescribed above in relation to FIGS. 5 and 6, when a blocking eventoccurs, the inertial mass (72) pivots in the direction (Y) causing theupper end of the lever (74) to pivot in direction (Z) thus causing thelatching ledges (86, 88) on the lever (74) and actuating shaft (21) todisengage. The strong spring (92), which is axially fixed at its lowerend to an increased diameter portion (21 c) and is axially fixed at itsupper end to the driving gear (12), acts to pull the increased diameterportion (21 c) of the actuating shaft, upwardly and so pulls theactuating shaft upwardly into the position shown in FIG. 8. Thismovement of the actuating shaft (21) moves the weaker springs (110)radially inwardly of the locking balls (25). In this way, the radiallyoutwardly biasing force on the locking balls (25) from the springs (110)is significantly reduced, as compared from the biasing force from thesprings (114) and the locking balls move radially inwardly, out of thepockets (13 a) in the cylindrical sleeve (13 a) of the driven gear (13).Thus, rotary drive to the driven gear (13) and so to the spindle (40)via the spindle drive shaft (5) is cut off. Accordingly, as soon as thelatching ledges (86, 88) of the lever (74) and clutch actuating shaft(21) are disengaged no further rotary drive is transmitted via theclutch arrangement (14) to the spindle (40). In this way the potentiallydangerous consequences of a blocking event are avoided.

It should be noted that with modification to the actuating shaft (21)the clutch arrangements of FIGS. 2 and 3 are suitable for use in thehammer of FIG. 5 and that the clutch arrangements of FIGS. 7 and 8 aresuitable for use in the hammer of FIG. 1.

FIGS. 9 and 10 show a further embodiment of a clutch arrangementsuitable for use in the hammer of FIG. 1, if a lower portion of theactuating shaft (21) is made of a magnetic element. The FIG. 9 and 10embodiment is also suitable for use in the arrangement of FIG. 5, if aspring arrangement is added for biasing the actuating shaft into anupper position.

The drive shaft (5) is formed with a pinion (7) at its upper end formeshing engagement with spindle drive gear (10). The shaft is rotatablymounted within the housing via bearings (23) and (15). The drive shaft(5) is hollow and the actuating shaft (21) is mounted within the driveshaft so as to be axially slideable within the drive shaft (5), with thelower end of the actuating shaft extending beyond the end of the driveshaft (5) remote from the pinion (7). The driving gear (12) is rotatablymounted on the drive shaft (5).

A first small diameter driven gear (13 c) is mounted on the drive shaft(5) for selective rotation therewith, depending on the position of theactuating shaft (21). A first set of clutch balls (25 a) are locatedwithin an associated set of through holes (103 a) in the driving gear(12), which through holes are radially inwardly of a second set ofthrough holes (103 b). A conical spring (107) biases the clutch balls(25) axially downwardly, towards the driven gears (13 c, 13 d) via awasher (105). The spring extends from its radially inner end, whichbears against a shoulder formed on the drive shaft (5) to a radiallyouter end which bears against the washer (105). The washer (105) islocated with a cooperating annular recess formed in the upper side ofthe driving gear (12). The spring (107) biases each of the first set offour clutch balls (25 a) into one of a set of four pockets (109) formedin the upper surface of the small diameter driven gear (13 c). In thisway, below a first predetermined torque, the first set of clutch balls(25 a) transmit rotatary drive from the driving gear to the smalldiameter driven gear (13 c). Above the first predetermined torque, thefirst set of clutch balls (25 a) will ride out and over the pockets(109) formed in the small diameter driven gear (13 c) and so will cutoff drive between the driving gear (12) and the small diameter drivengear (13 c). The rotary drive from the small diameter driven gear (13 c)can be transmitted to the drive shaft (5) depending on the position ofthe actuating shaft (21), as is described below.

A first pair of drive balls (113 a) are located within an associatedpair of upper holes (105 a) in the drive shaft. The drive balls areengageable with two of a set of four drive pockets (115 a) formed in theradially inner edge of the small diameter driven gear (13 c), torotatingly drive the drive shaft (5) when an increased diameter portion(121 a) is radially inwardly of the drive balls (113 a) and so pushesthe drive balls (113 a) into a radially outward position. When a reduceddiameter portion (121 b) of the actuating shaft (21) is radiallyinwardly of the drive balls (113 a) the drive balls can move radiallyinwardly and out of engagement with the drive pockets (115 a) of thesmall diameter driven gear (13 c) so that no rotary drive can betransmitted to the drive shaft (5).

A second large diameter driven gear (13 d) is mounted on the drive shaft(5) for selective rotation therewith, depending on the position of theactuating shaft (21). The second large diameter driven gear is locatedon the drive shaft (5) below and extends radially outwardly of the smalldiameter driven gear (13 a). A peripheral rim of the large diameterdriven gear (13 d) extends axially towards the driving gear (12) aroundthe periphery of the small diameter driven gear (13 c). A second set ofclutch balls (25 b) are located within an associated set of throughholes (103 b) in the driving gear (12), which through holes are radiallyoutwardly of the first set of through holes (103 a). The conical spring(107) biases each of the second set of four clutch balls (25 a), via thewasher (105), into one of a set of four pockets (111) formed in theupper surface of the peripheral rim of the large diameter driven gear(13 d). In this way, below a second predetermined torque, the second setof clutch balls (25 b) transmit rotatary drive from the driving gear tothe large diameter driven gear (13 b). Above the second predeterminedtorque, the second set of clutch balls (25 b) will ride out and over thepockets (111) formed in the large diameter driven gear (13 d) and sowill cut off drive between the driving gear (12) and the large diameterdriven gear (13 d). The second predetermined torque will be higher thanthe first due to the greater radial distance between the axis of thedrive shaft (5) and the second set of clutch balls (25 b) than theradial distance between the axis of the drive shaft and the first set ofclutch balls (25 a). The rotary drive from the large diameter drivengear (13 d) can be transmitted to the drive shaft (5) depending on theposition of the actuating shaft (21), as is described below.

A second pair of drive balls (113 b) are located within an associatedpair of lower holes (105 b) in the drive shaft. The drive balls areengageable with two of a set of four drive pockets (115 b) formed in theradially inner edge of the large diameter driven gear (13 d), torotatingly drive the drive shaft (5) when an increased diameter portion(121 a) of the actuating shaft is radially inwardly of the drive balls(113 b) and so pushes the drive balls (113 b) into a radially outwardposition. When a reduced diameter portion (121 b) of the actuating shaft(21) is radially inwardly of the drive balls (113 b) the drive balls canmove radially inwardly and out of engagement with the drive pockets (115b) of the large diameter driven gear (13 d) so that no rotary drive canbe transmitted to the drive shaft (5).

In a first position of the actuating shaft (21) of the clutch of FIGS. 9and 10 shown on the right hand side of FIG. 9, only the small diameterdriven gear (13 c) can rotatingly drive the drive shaft (5) via thefirst set of drive balls (113 a). The drive balls (113 a) in the firstposition as shown in the right hand side of FIG. 9 are urged intoengagement with the drive pockets (115 a) of the small diameter drivengear (13 c). This is because the increased diameter portion (121 a) ofthe actuating shaft is radially inward of the first set of drive balls(113) and so urge the drive balls radially outwardly. The second set ofdrive balls (113 b) are able to move radially inwardly into the reduceddiameter portion (121 b) of the actuating shaft (21) and out of thedrive pockets (115 b) in the large diameter driven gear (13 d), and sono rotary drive can be transmitted between the large diameter drivengear (13 d) and the drive shaft (5). In this first position, of theclutch of FIGS. 9 and 10, below a first relatively low predeterminedtorque, the first set of clutch balls (25 a) transmit rotatary drivefrom the driving gear to the small diameter driven gear (13 c). Abovethe first predetermined torque, the first set of clutch balls (25 a)will ride out and over the pockets (109) formed in the small diameterdriven gear (13 c) and so will cut off drive between the driving gear(12) and the small diameter driven gear (13 c). Accordingly, in thefirst position, the clutch arrangement of FIGS. 9 and 10 acts as anoverload clutch which slips at a first relatively low predeterminedtorque.

In a second position of the actuating shaft (21) of the clutch of FIGS.9 and 10 shown on the left hand side of FIG. 9, both the small diameterdriven gear (13 c) and the large diameter driving gear (13 d) canrotatingly drive the drive shaft (5) via the first and second sets ofdrive balls (113 a, 113 b). The drive balls (113 a, 113 b) in the secondposition as shown in the left hand side of FIG. 9 are urged intoengagement with the drive pockets (115 a, 115 b) of the small diameterdriven gear (13 c) and of the large diameter driven gear (13 d). This isbecause the increased diameter portion (121 a) of the actuating shaft isradially inward of the both sets of drive balls (113 a, 113 b) and sourge both sets of drive balls radially outwardly. In this secondposition, of the clutch of FIGS. 9 and 10, below a second predeterminedtorque, higher than the first predetermined torque, each sets of clutchballs (25 a, 25 b) transmit rotatary drive from the driving gear (12) totheir associated driven gear (13 c, 13 d). Above the secondpredetermined torque, the clutch balls (25 a, 25 b) will ride out andover the pockets (109, 111) formed in the associated driven gears (13 c,13 d) and so will cut off drive between the driving gear (12) and theassociated driven gears (13 c, 13 d). Accordingly, in the secondposition, the clutch arrangement of FIGS. 9 and 10 acts as an overloadclutch which slips at a second predetermined torque, which is higherthan the first.

To move between the first and second position of the clutch of FIGS. 9and 10 the actuating shaft is moved downwardly in the direction of arrow(W). This can be facilitated, for example as is shown in FIG. 11 a, byconnecting the lower end of the actuating shaft (21 c) to a linkage(120), which linkage is operated via a knob (122) actuatable by a userof the hammer to slideably move the actuating shaft (21) within thedrive shaft (5) adjust the slipping torque of the overload clutchbetween the first and second predetermined torques. In the arrangementof FIG. 11 a, an eccentric pin (122 a) acts to pull the linkage (120)upwardly and to thereby pull the actuating shaft (21) upwardly from itsposition in FIG. 11 a, against the biasing force of a spring (124) onrotation of the knob (122) out of the position shown in FIG. 11 a. Onmovement of the knob (122) back into the position shown in FIG. 11 a,the spring (124) returns the linkage and thus the actuating shaft to theposition shown in FIG. 11 a. The FIG. 11 a position would be the highertorque position shown in the left hand side of FIG. 9 and the linkage(120) and actuating shaft (21) would be pulled upwardly out of the FIG.11 a position into the lower torque position shown in the right handside of FIG. 9. Alternatively, the lower end. of the actuating shaft(21) could be connected directly to the knob.

The clutch arrangement of FIGS. 9 and 10 has a third position in whichthe reduced diameter portion (21 b) of the actuating shaft is radiallyinwardly of both sets of drive balls (113 a, 113 b). Thus, the driveballs are able to move radially inwardly and out of engagement with thedrive pockets (115 a, 115 b) of the driven gears (13 c, 13 d) and norotary drive can be transmitted between the driven gears (13 c, 13 d)and the drive shaft. In this third position, in which the actuatingshaft (21) is moved upwardly, in the opposite direction to the arrow (W)from the low torque position shown in the right hand side of FIG. 9, therotary drive to the drive shaft (5) and thus to the spindle (40) is cutoff.

The clutch arrangement of FIG. 9 and 10 can be moved into the thirdposition via a mode change linkage (126), shown in FIGS. 11 a and 11 b.The mode change linkage can be actuated between it positions in FIGS. 11a and 11 b by a mode change knob actuable by a user of the hammer. Inthe FIG. 11 a position the mode change linkage (126) is out ofengagement with the actuating shaft (21), this position would be adrilling only or a rotary hammering position of the mode change linkage.Thus the linkage (126), maintained in the position of FIG. 11 a by aspring (128) does not interfere with the arrangement for altering thepredetermined torque, discussed above in relation to FIG. 11 a, in thedrilling and/or rotary hammering modes. In the FIG. 11 b position, thelinkage (126) has been moved, against the biasing force of the spring(128) by a mode change knob into its hammering only mode position, inwhich the linkage (126) engages the lower portion (21 c) of theactuating shaft (21) to move it upwardly from the position shown in theright hand side of FIG. 9. This cuts off drive form the driving gear(12) to the drive shaft (5) and so there is no rotational output of thespindle (40) or the bit (68) mounted therein. The linkage arrangement(120, 122) for switching between low torque and high torque position inrotary modes of the hammer does not interfere with the operation of themode change linkage (126) to move the hammer into its non-rotary mode.On return of the mode change knob to a rotary mode position the biasingforce of the spring (128) will return the mode change linkage (126) toits position of FIG. 11 a.

The third position of the clutch arrangement can also be used to cut offrotary drive to the spindle (40) when a blocking event is detected. Ifthe blocking event is detected electronically, then an electromagnetsurrounding the lower portion of the actuating shaft (21) can beenergised to react against a magnetic element fitted to the lowerportion of the actuating shaft and to move the actuating shaft upwardlyinto its third position against the biasing forces of the springs (124)and (128). It should be noted that neither the arrangement (120, 122)for switching between the first and second positions, not the modechange linkage arrangement (126, 128) for switching to the thirdposition hinder the movement of the actuating shaft to its upperposition in response to the energisation of the electromagnet.

As an alternative to an electromagnet, the mechanical arrangement fordetecting blocking events of FIG. 6, could be used in conjunction withthe clutch arrangement of FIGS. 9 and 10. Here the latching ledge (86)of the linkage (74) would engage the lower portion (21 c) of theactuating shaft (21) and the shaft would be biased to move upwardly intothe third position should the latching ledge and lower portion (21 c)become disengaged in the event of a blocking event.

FIG. 12 shows an additional design of overload clutch (14) which can beused for selectively cutting of rotary drive between the driving gear(12) and the driven gear (13) for mode change or in response to ablocking event The driving gear (12) is rotatably mounted on the driveshaft (5) and has clutch balls (25) mounted in holes extending axiallythrough the driving gear. A driven gear (13) is non-rotatably mounted onthe drive shaft (5) and is formed with a set of pockets (13 b) on itsface facing the driving gear (12) for receiving the clutch balls (25). Aconical spring (107) urges the driven gear (13) towards the driving gear(12). An actuating ring (130) is located below the driving gear (12) andin a first position shown on FIG. 12, the ring (130) pushes the clutchballs (25) into engagement with the pockets (13 b) in the driven gear(13). The actuating ring (130) can be moved downwardly in direction (V)into a position in which it no longer urges the clutch balls intoengagement with the pockets (13 b) in the driven gear (13).

In the position shown in FIG. 12 the clutch acts as an overload clutch.Below a predetermined torque, the conical spring (107) biases the clutchballs (25) into engagement with the pockets (13 b) in the driven gear(13) to thereby transmit rotation from the driving gear (12) to thedriven gear (13). Thus, rotary drive is transmitted to the spindle (40)via the spindle drive shaft (5). However, above the predeterminedtorque, the biasing force from the conical spring (107) becomesinsufficient to bias the clutch balls (25) into the pockets (13 b) inthe driven gear (13) and the driven gear (13) can move axially againstthe force of the spring (107) to ride over clutch balls (25). Thus, thedriven gear (13) rotates with respect to the driving gear (12) androtary drive to the spindle drive shaft (5) and thus to the spindle (40)is cut-off. When the actuating ring is moved to the second position norotary drive can be transmitted between the driving gear (12) and thedriven gear (13) because the clutch balls (25) cannot engage the pockets(13 b) in the driven fear (13). Therefore, on detection of a blockingevent the guide ring is moved to the second position to disengage rotarydrive. This can be done by mechanical or electromechanical means, asdescribed above in relation to FIGS. 1, 5 and 6. Additionally oralternatively, the locking ring (130) can be moved to the secondposition by a mode change arrangement on switching to a non-rotary modeof the hammer.

Referring now to FIG. 13, a hammer of a further embodiment of theinvention comprises an electric motor (2), a spindle drive train (4) anda crank drive arrangement (6) surrounded by a plastic housing (8). Arear handle (10) and a trigger switch arrangement (not shown) are fittedto the rear of the housing (8). An electric cable (not shown) extendsthrough a cable guide and connects the motor (2) to an externalelectricity supply. Thus, when the cable is connected to the electricitysupply and the trigger switch arrangement is depressed, the motor (2) isactuated to rotationally drive the armature of the motor (2).

A main driving gear (12) is press fitted onto a motor pinion (14) andhas teeth which engage the teeth of a driving gear (16) of an overloadclutch arrangement (18) to rotatingly drive the driving gear (16). Thedriving gear (16) rotatingly drives a bevel gear (20) of the overloadclutch arrangement (18) when the torque transmitted between the twogears (16, 20) is below predetermined thresholds and if no blockingevent is detected. The bevel gear (20) meshes with a beveled spindledrive gear (22) which beveled spindle drive gear (22) is rotatablymounted on a cylindrical hollow spindle (40) and can freely rotate aboutthe spindle. The beveled spindle drive gear (22) rotatingly drives thespindle (40) via a rotary drive clutch described below. The overloadclutch arrangement (18) is described in more detail below.

The teeth of the driving gear (12) also engage the teeth of a crankdrive gear (24) to rotatingly drive the crank drive gear (24). The crankdrive gear (24) is non-rotatably mounted on a crank drive spindle (26).A crank plate (30) is non-rotatably mounted at the end of the drivespindle (26) remote from the crank drive gear (24), which crank-plate(30) is formed with an eccentric bore for housing an eccentric crank pin(32). The crank pin (32) extends from the crank plate (30) into a boreat the rearward end of a con-rod or crank arm (34) so that the con-rod(34) can pivot about the crank pin (32). The opposite forward end of thecon-rod (34) is formed with a bore through which extends a trunnion pin(36) so that the con-rod (34) can pivot about the trunnion pin. Thetrunnion pin (36) is fitted to the rear of a piston (38) by fitting theends of the trunnion pin (36) into receiving bores formed in a pair ofopposing arms (42), which arms extend to the rear of the piston (38).The piston (38) is reciprocally mounted in the cylindrical hollowspindle (40) so that it can reciprocate within the hollow spindle. AnO-ring seal (44) is fitted in an annular recess formed in the peripheryof the piston (38) so as to form an air tight seal between the piston(38) and the internal surface of the hollow spindle (40).

Thus, when the motor (2) is actuated, the armature pinion (14)rotatingly drives the main driving gear (12) and the main driving gear(12) rotatingly drives the crank drive spindle (26) via the crank drivegear (24). The drive spindle (26) rotatingly drives the crank plate (30)and the crank arm arrangement comprising the crank pin (32), the con-rod(34) and the trunnion pin (36) convert the rotational drive from thecrank plate (30) to a reciprocating drive to the piston (38). In thisway the piston (38) is reciprocatingly driven back and forth along thehollow spindle (40), when the motor (2) is actuated by depression of thetrigger switch (not shown) on rear handle (10). The main driving gear(12) also drives the driving gear (16) of the clutch arrangement (18)which drives the bevel gear (20) of the clutch arrangement. The bevelgear (20) of the clutch arrangement rotatingly drives the spindle drivegear (22) and thus the spindle (40) when the spindle drive gear (22) isdrivingly connected to the spindle (40). When the mechanism by which thespindle drive gear (22) is connected to the spindle (40) is connected,the hammer operates in a chisel and drill mode, and when it isdisconnected, the hammer operates in a chisel mode only.

A two torque clutch of the clutch arrangement (18) of the hammer of FIG.13 will now be described in more detail with reference to FIGS. 14 to17.

The bevel gear (20) which forms part of the clutch arrangement (18) isintegrally formed with a shaft (100) of circular cross section. Theupper end of the shaft (100) is rotatably mounted within the housing (8)of the hammer via a bearing comprising an inner race (102) which isrigidly attached to the shaft (100), an outer race (104) which isrigidly attached to the housing and ball bearings (106) which allows theouter race (102) to freely rotate about the inner race (102). Thebearing is located adjacent the underside of the bevel gear (20).

The driving gear (16) is rotatably mounted on the shaft (100) and canfreely rotate about the shaft (100). The driving gear (16) abuts theunderside of the inner race (102) of the bearing and is prevented fromaxially sliding away from (downwardly) by the rest of the clutchmechanism which is described in more detail below.

The driving gear (16) is so shaped that it surrounds a toroidal space,the space being surrounded by a flat bottom (108) which projectsradially outwards from the shaft (100), an outer side wall (110) uponthe outer surface of which are formed the teeth of the driving gear (16)and an inner side wall (112) which is adjacent the shaft (100).

Located within the toroidal space of the driving gear (12) adjacent theflat bottom (108) is a washer (114) which surrounds the inner wall 112and shaft (100). Mounted on top of the washer (114) is belleville washer(116). The inner edge of the belleville washer is located under theinner race (102) of the bearing whilst the outer edge of the bellevillewasher abuts against the outer edge of the washer (114) adjacent theouter wall (110) of the driving gear (16). The driving gear (112) isheld axially on the longitudinal axis of the shaft (100) in relation tothe belleville washer so that the belleville washer (116) is compressedcausing it to impart a downward biasing force onto the washer (114)towards the flat bottom (108) of the driving gear (16).

Formed in the flat bottom (108) of the driving gear (16) are two sets ofholes; a first inner set (118) of five, each located equidistantly fromthe longitudinally axis of the shaft in a radial direction and angularlyfrom each other around the longitudinal axis of the shaft (100); asecond outer set (120) of five, each located equidistantly from thelongitudinal axis of the shaft in a radial direction and angularly fromeach other around the longitudinal axis of the shaft (100). The radialdistance of the outer set (120) from the longitudinal axis of the shaft(100) is greater than that of the inner set (118).

A ball bearing (122) is located in each of the holes and abuts againstthe underside of the washer (114). The diameters of all the ballbearings (122) are the same, the diameter being greater than thethickness of the flat bottom (108) of the driving gear (16) therebyresulting either the top or bottom of the ball bearings (122) protrudingbeyond the upper or lower surfaces of the flat bottom (108) of thedriving gear (16).

Mounted on the spindle (100) below and adjacent to the driving gear (16)is a first slip washer (124). The first slip washer (124) comprises acircular hole (123) with two splines (125) projecting into the hole(123) which, when the washer is mounted on the spindle (100), locatewithin two corresponding slots (127) formed in the spindle (100). Assuch, the first slip washer (124) is none rotatably mounted on thespindle, the spindle (100) rotating when the first slip washer (124)rotates.

Referring to FIGS. 16A to 16C, formed on one side of the first slipwasher (124) around the periphery is a trough (126) with a U shapedcross section (see FIG. 16B). The circular trough (126) is separatedinto five sections (128), the depth of each section (128) of troughvarying from a low point (129) to high point (131). Each section (128)of trough (126) is the same in shape as the other sections (128) oftrough (126). The low point (129) of one section (128) of trough isadjacent to the high point (131) of the next section as shown in FIG.16C. The two are connected via a ramp (134). When the slip washer (124)is mounted on the shaft (100), the side of the first slip washer (124)faces the driving gear (16). The diameter of the first slip washer (124)is less than that of the driving gear (16) and is such that, when theslip washer (124) is mounted on the shaft (100), the trough (126) facesthe inner set of holes (118). The five sections (128) which form thetrough (126) correspond to the five holes (118) which formed theinnermost set of holes in the driving gear (16) so that, when the clutch(18) is assembled, one ball bearing (122) locates in each section (128)and trough (126).

Mounted on the spindle (100) below the first slip washer (124).is asecond slip washer (140). The second slip washer (146) is dish shapedhaving an angled side wall (142) surrounding a flat base (144). Whenmounted on the spindle, the first slip washer (124) locates within thespace surrounded by the side wall (142) and the flat base (144) surfaceas best seen in FIG. 14. The second slip washer (140) can freely rotateabout the spindle (100). A rectangular slot (146) superimposed on acircular hole (147) is formed in the flat base (144) symmetrical aboutthe axis of rotation of the second slip washer (140). Formed on the topof the angled side wall (142) is a flange (148) which projects radiallyoutwards.

Referring to FIG. 17, formed on the top side of the radial flange (148),around the radial flange (148), is a trough (150) with a U shaped crosssection which is similar in shape to that on the first slip washer(124). The circular trough (150) is separated into five sections (151),the depth of each section of trough varying from a low point (152) to ahigh point (154). Each section (151) of the trough (150) is the same inshape as the other sections of trough. The low point (152) of onesection of trough is adjacent to the high point (154) of the nextsection. The two are connected via a ramp (156). When the second slipwasher (140) is mounted on the shaft as shown, the side of the flange(148) with the trough (150) faces the driving gear (16). The diameter ofthe flange (150) is such that, when the second slip washer (140) ismounted on the shaft (100), the trough (150) faces the outer set ofholes (120) in the driving gear (16). The five sections (151) which formthe trough (150) correspond to the five holes (120) which form theoutermost set of holes in the driving gear (16) so that, when the clutchis assembled, one ball bearing (122) locates in each section of thetrough (150).

The size of the ramps (134) in the trough (126) of the first slip ring(124) is less than that of the size of the ramps (156) formed in thetrough (150) of the second slip washer (140), the variation of theheight of each section of trough in the first slip washer (124) from thelow end (120) to the high end (131) being less than that of thevariation of the height of each section of trough in the second slipwasher from the low end (152) to the high end (154).

When the clutch is assembled, the ball bearings (122) in the innermostset of holes (118) in the driving gear (16) locate within the trough(126) of the first slip washer (124) (one ball bearing per section) andthe ball bearings (122) in the outer most set of holes (120) in thedriving gear (16) locate within the trough (150) of the second slipwasher (140) (one ball bearing per section).

A circular clip (160) is rigidly mounted on the spindle (100) below thesecond slip washer (140) which holds the first and second slip washers(124, 140) together with the driving gear (16) against the underside ofthe bearing in a sandwich construction preventing axial displacement ofthe three along the spindle. Rotation of the circular clip results inrotation of the spindle (100).

The lower end of shaft (100) is rotatably mounted within the housing (8)of the hammer via a second bearing comprising an inner race (170) whichis rigidly attached to the shaft (100), an outer race (172) which isrigidly attached to the housing (8) and ball bearings (174) which allowsthe outer race (174) to freely rotate about the inner race (170). Thebearing is located adjacent the underside of the circular clip (160).

When the clutch is fully assembled and no rotary torque is beingtransferred through it, each of the ball bearings in the innermost holes(118) of the driving gear (16) locate in the lowest points (134) of thecorresponding sections (128) of the trough in the first slip washer(124) as indicated by the dashed line (180). When the ball bearings(122) are located within the lowest points (134) of the sections (128)of the trough (126), the tops of the ball bearings (122), which areadjacent to the washer (114), are flush with the surface facing thewasher (114) of the flat bottom (108) of the driving gear (16). The ballbearings (122) locate in the lowest points (134) due to the biasingforce of the belleville washer (116) which is biasing the washer (114)in a downward direction which in turn pushed the ball bearings (122) totheir lowest positions.

Similarly, when the clutch is fully assembled and no rotary torque isbeing transferred through it, each of the ball hearings (122) in theoutermost holes (120) of the driving gear (16) locate in the lowestpoints (156) of the corresponding sections (151) of the trough (150) inthe second slip washer (140) as indicated by the dashed line (182). Whenthe ball bearings (122) are located within the lowest point (156) of thesections (151) of the trough (150), the tops of the ball bearings (122),which are adjacent to the washer (114), are flush with the surface ofthe flat bottom (108) of the driving gear (16) facing the washer (114).The ball bearings (122) locate in the lowest points (156) due to thebiasing force of the belleville washer (116) which is biasing the washer(114) in a downward direction which in turn pushes the ball bearings(122) to their lowest positions.

Formed through the length of the spindle (100) is a tubular passageway(186). Located within the lower section of the tubular passageway (186)is a rod (188). The rod projects below the spindle (100) beyond thespindle (100). A seal (189) is attached to the base of the spindle (100)and surrounds the rod (188). The seal (189) prevents the ingress ofdirt.

Rigidly attached to the upper end of the rod (188) a sleeve (190).Projecting in opposite directions perpendicularly to the sleeve (190)are two pegs (192). The sleeve (190) located within the spindle (100) ina position along the length of the spindle (100) where the sleeve (190)and pegs (192) are surrounded by the circular clip (160). Two verticalslots (194) are formed in the sides of the circular clip (160). The topend of the slots (194) extends to the top of the circular clip (160).The bottoms of the slots (194) extends part way down the circular clip(160), terminating in a base. In each of the slots (194) is located oneof the pegs (192). The pegs (192) extend through the slots (194, 127) onthe spindle (100) and the circular clip (160). The rod (188), togetherwith the sleeve (140) and two pegs (192) can vertically slide up anddown. The lowest position is where the two pegs (192) abut the bottom ofthe slots (194) of the circular clip (160), further downward movementbeing prevented by the base of the slots (194) in the circular clip asshown in FIG. 14. The highest position is where the two pegs (192)locate within the rectangular slot (146) within the second slip washer(140) in addition to being located within the top end of the slot (160),further upward movement being prevented by the underside of the firstslip washer (124). A spring (196) locates between the top of the spindle(100) and the sleeve (190) in the upper section of the tubularpassageway (186). The spring biases (196) the sleeve (190), two pegs(192) and rod (188) towards their lowest position. Regardless of whetherthe pegs (192) are at their upper or lower position, rotation of thepegs (192), results in rotation of the circular clip (160) due to thepegs (192) being located in the slots (194) which in turn results inrotation of the spindle (100).

Movement of the rod (188) between its lowest and highest positionchanges the clutch (18) from a low torque to a high torque clutch. Themechanism by which the rod is moved vertically is described below. Theclutch operates by transferring the rotary movement from the drivinggear (16) to the bevel gear (20) which is integral with the spindle(100). When the torque across the clutch (18) is below a predeterminedvalue the driving gear (16) will rotatingly drive the bevel gear (20).When the torque across the clutch is above a predetermined value, thedriving gear (16) will rotate but the bevel gear (20) will remainstationary, the clutch (18) slipping as the driving gear (16) rotates.The predetermined value of the torque at which the clutch (18) slips canbe alternated between two preset values by the sliding movement of therod (188) between the lowest and highest positions.

The mechanism by which the clutch (18) works will now be described.

Low Torque Operation

The rod (188) is located in its lowest position when the clutch (18) isacting as a low torque clutch. When in this position, the pegs (192) aredisengaged from the rectangular aperture (146) in the second slip washer(140). As such, therefore, the second slip washer (140) can freelyrotate about the spindle (100). As such no rotary movement can betransferred between the second slip washer (140) and the spindle (100).Therefore, all rotary movement between the driving gear (16) and thebevel gear (20) is transferred via the first slip washer (124) only.

The electric motor (2) rotatingly drives the driving gear (16) via themain driving gear (12). The driving gear (16) can freely rotate aboutthe spindle (100). As such, no rotary movement can be transferred to thespindle (100) directly from the driving gear (16). As the driving gearrotates, the ball bearings (122) located within the innermost set ofholes (118) formed within the driving gear (16) also rotate with thedriving gear (16). Under normal circumstances when the rotary movementis being transferred, the ball bearings (122) are held in the lowestpoint (134) of the section (128) of the trough (126) formed in the firstslip washer (124) by the washer (114) which is biased downwardly by thebiasing force of the belleville washer (116). The direction of rotationis such that the ball bearings (122) are pushed against the ramps (134)of the trough (126), the ball bearings (122) being prevented from ridingup the ramps (134) by the biasing force of the belleville washer (116).As such, when the ball bearings in the innermost set (118) rotate, theramps (134) and hence the first slip washer (124) also rotate. As thefirst slip washer (124) is non-rotatably mounted on the spindle (100)due to the splines (125) engaging the slot (127) in the spindle (100),as the first slip washer (126) rotates, so does the spindle (100) andhence the bevel gear (20). As such the rotary movement is transferredfrom the driving gear (16) to the bevel gear (22) via the ball bearings(122) in the innermost set of holes (118), the ramps (134) and the firstslip washer (124).

However, when a torque is applied to the clutch (18) (in the form of aresistance to the turning movement of the bevel gear (22)) above acertain amount, the amount of the force required to be transferred tofrom the ball bearings (122) to the ramps (134) on the first slip washer(124) is greater than the force exerted by the belleville (116) on theball bearings (122) keeping them in the lowest point (129) of thesection (128) of the trough (126). Therefore, the ball bearings (122)ride over the ramps (134) and then continue down the slope of the nextsection (128) until it engages the next ramp (134). If the torque isstill greater than the predetermined amount the process is repeated, theball bearing (122) riding up the ramps (134) against the biasing forceof the belleville washer (116) and then rolling across the next section.As this happens the first slip washer (124) remains stationary and hencethe spindle (100) and bevel gear (22) also remain stationary. Therefore,the rotary movement of the driving gear (16) is not transferred to thebevel gear (22).

Though the second slip washer (140) plays no part in transferring therotary movement of the driving gear (16) to the spindle (100) in the lowtorque setting, it is nevertheless rotated by the driving gear (16).

High Torque Operation

The rod (188) is located in its highest position when the clutch (18) isacting as a high torque clutch. When in this position, the pegs (192)are engaged with the rectangular aperture (146) in the second slipwasher (124). As such, the second slip washer (124) is rotatably fixedto the spindle (100) via the pegs (192) located in the rectangular slot(146), the slots (194,127) of the circular clip (160) and spindle (100).As such rotary movement can be transferred between the second slipwasher (140) and the spindle (100). Therefore, rotary movement betweenthe driving gear (16) and the bevel gear (22) can be transferred via thefirst slip washer (124) and/or the second slip washer (140).

The mechanism by which the driving gear (16) transfers its rotary motionto the first slip washer (124) via the ball bearings (122) and ramps(134) is the same as that for the second slip washer (140).

The electric motor (2) rotatingly drives the driving gear (16) via themain driving gear (12). The driving gear (16) can freely rotate aboutthe spindle (100). As such, no rotary movement can be transferred to thespindle (100) directly from the driving gear (16). As the driving gear(16) rotates, the ball bearings (122) located within the innermost (118)and outermost (120) set of holes formed within the driving gear (16)also rotate with the driving gear (16). Under normal circumstances whenthe rotary movement is being transferred, the ball bearings (122) areheld in the lowest point (129, 152) of the sections (128, 151) of thetroughs (126, 150) formed in both the first slip washer (126) and thesecond slip washer (140) by the washer (114) which is biased downwardlyby the biasing force of the belleville washer (116). The direction ofrotation is such that the ball bearings (122) are pushed against theramps (134, 156) of the troughs (126, 150) of both the first slip washer(124) and the second slip washer (140), the ball bearings (122) beingprevented from riding up the ramps (134, 156) by the biasing force ofthe belleville washer (116). As such, when the ball bearings (122)rotate, the ramps (134, 156) and hence the first and second slip washers(124, 140) also rotate. As both the first and second slip washers (124,140) are non-rotatably mounted on the spindle (100), as the first andsecond slip washers rotate (124, 140), so does the spindle (100) andhence the bevel gear (22). As such the rotary movement is transferredfrom the driving gear (16) to the bevel gear (22) via the ball bearings(122) in the inner and outermost set of holes (118, 120), the ramps(134, 156) and the first and second slip washers (124, 140).

However, when a torque is applied to the clutch (18) (in the form of aresistance to the turn movement of the bevel gear (22)) above a certainamount, the amount of the force required to be transferred to from theball bearings (122) to the ramps (134, 156) is greater than the forceexerted by the belleville washer (116) on the ball bearings (122)keeping it in the lowest points (129, 152) of the sections of thetroughs. The amount of torque required in the high torque setting ishigher than that in the low torque setting. This is due to the size ofthe ramps (156) between sections (151) of the trough (150) in the secondslip washer (140) being greater than the size of the ramps (134) betweensections (128) of the trough (126) in the first slip washer (124)requiring the belleville washer (116) to be compressed to a greaterextent and hence requires force for it to be done so. Therefore, whenthe force exceeds this greater value, the ball bearings (122) ride overthe ramps (134, 156) and then continue down the slope of the nextsection until they engage the next ramp (134, 156). If the torque isstill greater than the predetermined value the process is repeated, theball bearings (122) riding up the ramps (134, 156) against the biasingforce of the belleville washer (116) and then rolling across the nextsection. As this happens the first and second slip washers (124, 140)remain stationary and hence the spindle (100) and bevel gear (22) alsoremain stationary. Therefore, the rotary movement of the driving gear(16) is not transferred to the bevel gear (22).

Torque Change Mechanism

The mechanism by which the torque setting of the clutch (18) is adjustedwill now be described.

Referring to FIGS. 14 and 18, the underside of the two torque clutch(18) is enclosed within a clutch housing (200). The rod (188) projectsthrough the base of the housing (200). The lowest end of the rod engageswith a cam (202). The cam (202) is mounted on a shaft (204) which canpivot about its longitudinal axis (206). The rod (186) and hence the cam(200) are biased towards their lowest position by the spring (196)within the spindle (100) of the clutch (18). Pivotal movement of theshaft (204) results in a pivotal movement of the cam (202) which causesthe end of the rod (188) slidably engaged with the cam (202) to ride upthe cam (202) causing the rod (188) to slide vertically upwards againstthe biasing force of the spring (196) changing the clutch (18) from thelow torque to high torque setting.

Attached to shaft (204) is a flexible lever (208). Attached to the endof the flexible lever (208) is the cable (210) of a bowden cable (212).The pulling movement of the cable (210) pulls the lever (208) causing itand the shaft (204) to rotate about the axis (206). This results in thecam (202) pivoting which in turn moves the rod (188) vertically upwards.Release of the cable (210) allows the lever (208) and shaft (204) topivot, allowing the cam (202) to move to its lowest position due to thebiasing force of the spring (196) via the rod (188). The flexible lever(208), is sufficiently stiff to be able to move the shaft (204) andhence the cam (202) to change the torque setting of the clutch (18).However, if the two pegs (192) are not aligned with rectangular aperture(146) on the second slip washer (140), the pegs (192) and hence the rod(188) is prevented from travelling to their uppermost position. However,the means by which the cable (210) is pulled will not be able to discernthis. Therefore, in this situation, the lever (208) bends allowing thepegs (192) to abut the underside of the second slip washer (140) whilstallowing the cable (210) to be pulled by its maximum amount. When themotor (2) is energised, the second slip washer (140) will rotate,aligning the pegs (192) with the rectangular hole (146), at which pointthe pegs (192) enter the rectangular hole (146) due to the biasing forceof the bent lever (208).

Referring to FIGS. 18 and 19, the bowden cable (212) wraps around theexternal wall (214) of the motor (2) to the rear of the body (8) of thehammer. Mounted at the rear of the body (8) facing the rear handle (10)(see FIG. 14) is a pivotal finger grip (216) which is capable ofpivoting about a vertical axis (218). The cable (210) of the bowdencable (212) is attached to the pivoted finger grip (216). The sleeve ofthe bowden cable (212) is fixed at both ends to the housing (8). Thuspivotal movement of the finger grip (216) pulls the cable (210) throughthe sleeve thus pulling the lever (208). The spring (196) in the clutchpulls the cable (210) via the rod (188), the cam (202) and lever (208)which in turn pulls the pivotal finger grip (216) to a first position.The finger grip (216) can be pushed to a second position against thebiasing force of the spring as it pulls the cable (210) of the bowdencable. Thus pivotal movement of the finger grip (216) moves the clutch(18) from the low torque position to the high torque setting. Release ofthe finger grip (216) when located in the second position (clutch 18 isthe high torque position) allows it to travel to its first position dueto the biasing force of the spring (196) as it pushes the rod (188) andhence cam (202) downwardly.

The latch mechanism for the finger grip (216) in the high torqueposition will now be described.

Mounted below the finger grip (216) is a vertical lever (220). Thevertical lever is mounted on the body (8) of the hammer via a horizontalshaft (222). The shaft (222), and hence the vertical lever (220), canpivot about a horizontal axis (224) from a first position where thevertical lever (220) is vertical to a second position where the topvertical of lever (220) points away from inside the body (8) towards therear handle (10).

Referring to FIG. 20, formed on the top end of the vertical shaft (220)is a hump (226). Mounted below the finger grip (216) is a leaf spring(228) which is suspended across two arms (230) which projecthorizontally. When the finger grip (216) is pivoted about the verticalaxis (218), the two arms (230) move in a direction indicated by Arrow Ein FIG. 20. The leaf spring (228) has a link (232) formed within itwhich projects downwardly.

When the vertical lever (220) is in its normal operating position it isvertical. When the finger grip (216) is in its first position when theclutch (18) is in its low torque setting, the leaf spring (228) is tothe left of the lever (220) when viewed in FIG. 20.

When the clutch (18) is to be moved to the high torque setting, thefinger grip (216) is pivoted about the vertical axis (218), the leafspring moving towards the top of the lever (220). As it does so, a firstside (234) of the link (232) engages with the first side (236) of thehump (226) on the top of the lever (220). As the finger grip (216)continues to pivot, the leaf spring (228) flexes, the link (232) movingupwardly and over the hump (226) and then reverts to original shape witha second side (237) of the link (232) engaging the second side (238) ofthe hump (226), as shown in FIG. 20.

The biasing force of the spring (196) in the clutch pulls the fingergrip (216) in the direction of Arrow F in FIG. 20. However, the force ofthe spring (196) is insufficient to pull the leaf spring lock over thehump (226).

To move the clutch (18) from a high torque setting to a low torquesetting, the operator pushes the finger grip (216) forcing the leafspring (228) to ride back over the hump (226) after which the spring inthe clutch pulls the finger grip (216) to a position where the clutch isin a low torque setting.

It is desired to ensure that the two torque clutch reverts to the lowtorque setting when the electrical power is applied to the hammerforcing the operator to consciously move the clutch (18) to a hightorque setting when required.

Furthermore, it is desired to prevent operation of the hammer when thefinger grip has been moved to the low torque setting but the clutchremains in the high torque setting.

Referring to FIGS. 19 and 21, attached to the lower end of the verticallever (220) is a solenoid (250). The solenoid comprises a coil of wire(252) and a magnetic pin (254). A spring (256) is attached between thecasing (258) of the coil (252) and the pin (254) and biases the pin(254) into the coil (252). One end (260) of the pin (254) is attached tothe lower end of the vertical lever (220). The longitudinal axis of thepin (254) is horizontal.

When the solenoid is not activated by an electric current, the pin (254)is moved to an inward position by the force of the spring (256). Thismoves the end of the vertical lever (220) causing the lever (220) topivot to a position where the vertical lever (220) is vertical. In thisposition, the leaf spring (228) mounted below the finger, grip (216) canengage the hump (226) on the top end of the lever (220).

When the solenoid is activated by an electric current, the pin (254) ispulled into the coil (252) causing it to pull the lower end of the leverwhich in turn causes the lever (220) to pivot about the axis (224)resulting in the top end of the lever (220) with the hump (226) pivotingaway from the leaf spring (228) mounted below the finger grip (216)(pivoting out of the page as shown in FIG. 20). If the finger grip (216)is being held in its second position so that the clutch is in the hightorque setting, by the leaf spring (228) being held by the hump,activation of the solenoid (250) then pivots the lever and disengagesthe hump (226) from the leaf spring (228). This allows the finger grip(216) to return to its first position and hence allow the clutch (18) tomove to a low torque setting due to the biasing force of the spring(196) on the clutch.

A sensor (not shown) is mounted on the flexible lever (208) and detectsthe position of the end of the lever (208). A sensor (not shown) ismounted on the finger grip (216) and detects the position of the fingergrip (216). A sensor is mounted within the trigger switch and detectswhether a current is applied to the hammer. A circuit monitors the threesensors and based on a number of predetermined conditions activates thesolenoid (150), as shown in more detail in FIG. 22.

In normal operation the solenoid is not activated.

If no current is being supplied to the hammer (i.e. it is unplugged),the circuit monitors when a current is supplied to operate the hammer(i.e. the hammer is plugged in).

When the circuit detects the current, it checks that the two sensors onthe flexible lever (208) and the finger grip (216) both indicate thatthe clutch (18) is in the low torque setting. If they are, the circuitdoes nothing. If they are not or one of them is not, it activates thesolenoid ensuring that the finger grip can return to its lowestposition. Once the two sensors both indicate that the torque clutch isin the low torque setting, the circuit switches the solenoid off,allowing the finger grip to function as normal.

The circuit further constantly monitors the two sensors on the flexiblelever (208) and the finger grip (216). If the sensor on the finger grip(216) indicates it is the first setting but the sensor on the flexiblelever (208) indicates the clutch is in the high torque setting, itdeactivates the hammer, preventing use until the clutch is reset.

The sensor is placed on the flexible lever (208) rather than the clutch(18), because, if the pegs (192) are not aligned with the rectangularhole (146) in the second slip washer (140), the sensor may indicate thatthe clutch is in the low torque setting whereas the flexible lever (208)may be biasing it into a high torque setting and, when the hammer isoperated, would move the clutch (18) into the high torque setting.

It will be appreciated by persons skilled in the art that the aboveembodiments have been described by way of example only, and not in anylimitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims. For example, it will be appreciated that thetwo-torque clutch (18) described with reference to FIGS. 13 to 17 mayhave more than two torque settings.

1. A power tool comprising: a motor; a tool holder for holding an outputmember; a transmission for transmitting rotary torque from the motor tothe tool holder, the transmission comprising a first transmissionportion and a second transmission portion and a clutch assembly locatedbetween the first transmission portion and the second transmissionportion; the clutch assembly comprising: a clutch shaft; an output gearfor driving the second transmission portion with rotary torquetransmitted across the clutch assembly, the output gear rotationallyfixed relative to the clutch shaft; an input gear for receiving rotarytorque from the first transmission portion, the input gear locatedaround and rotatable relative to the clutch shaft; a first slip disc anda first clutch mechanism operable for transmitting rotary torque notexceeding a first predetermined torque limit from the input gear to theclutch shaft; a second slip disc and a second clutch mechanism operablefor transmitting rotary torque not exceeding a second predeterminedtorque limit from the input gear to the clutch shaft, the secondpredetermined torque limit being greater than the first predeterminedtorque limit; an actuator mechanism for selectively engaging the secondslip disc for transmitting rotary torque, not exceeding the secondpredetermined torque limit, from the input gear to the clutch shaft. 2.A power tool according to claim 1 wherein: the first slip disc surroundsthe clutch shaft and is nonrotatable and axially slidable relative tothe clutch shaft; the second slip disc surrounds and is rotatablerelative to the clutch shaft; and the actuator mechanism is movable to aposition where it locks the second slip disc in rotation with the clutchshaft.
 3. A power tool according to claim 2 wherein the first clutchmechanism comprises: a first plurality of holes in the input gear; afirst plurality of balls locatable in the first plurality of holes; afirst trough on a first side of the first slip disc, the first troughincluding a first series of recesses and ramps.
 4. A power toolaccording to claim 3 wherein the second clutch mechanism comprises: asecond plurality of holes in the input gear; a second plurality of ballslocatable in the second plurality of holes; a second trough on a firstside of the second slip disc, the second trough including a secondseries of recesses and ramps.
 5. A power tool according to claim 4wherein the first clutch mechanism and the second clutch mechanism arebiased into a torque transmitting arrangement by a single spring.